1. Field of the Invention
The present invention relates to a magnetic seal bearing unit used for example for a wafer rotation mechanism, wafer transfer mechanism, etc. of an oxidation-diffusion furnace, chemical vapor deposition (CVD) apparatus, etc. for rotatably holding a rotary shaft while maintaining air-tightness of a vacuum processing chamber and to a vacuum processing apparatus.
2. Description of the Related Art
In the past, for example, an oxidation-diffusion furnace or CVD apparatus or other semiconductor processing apparatus has been improved in the uniformity of the processing by rotating the susceptor, wafer holder, etc. holding the wafer inside the vacuum processing chamber kept at a high vacuum.
Also, a system called "cluster tools" is used to successively perform different processing while holding a high vacuum and thereby improve the throughput. In this system, a plurality of processing chambers are provided around a common transfer chamber. After a processed wafer is taken out from a processing chamber by an arm of the wafer transfer mechanism provided in the transfer chamber, the wafer is transferred to another processing chamber.
A rotation mechanism for high vacuum required for these apparatuses is required to have a high air-tightness and not to produce any wafer contaminants.
For example, Japanese Unexamined Publication (Kokai) No. 7-169706 discloses an upright heat treatment apparatus having a rotation mechanism providing a drive side magnet at an outer casing secured to a pulley connected to a motor shaft by a belt and providing a driven side magnet at a rotary shaft and causing rotation of a wafer holder connected to the rotary shaft by the repulsion force of the two magnets along with the rotation of the pulley.
In this rotation mechanism, an inner casing secured to the bottom surface of the furnace is interposed between the outer casing and the rotary shaft at a predetermined distance from the two. Ceramic bearings having ceramic rollers are provided at a total of four locations above and below the two magnets between the inner casing and the outer casing and between the inner casing and the rotary shaft.
Accordingly, particles are generated from the ceramic bearings or due to damage of magnets made of brittle materials or corrosion due to gas from the furnace. There is therefore the problem that the wafer is contaminated by heavy metals and organic substances.
Accordingly, the upright heat treatment apparatus disclosed in this publication is designed with a feed pipe and an exhaust pipe for an inert gas provided between the furnace and the rotation mechanism and with the inert gas fed and discharged by a control unit for example for 10 seconds before evacuation in the furnace. At this time, in the rotation mechanism having the above structure, it is necessary to discharge the air around the magnets and around the ceramic bearings beneath them. A communicating pipe is formed in the rotary shaft to discharge the air inside the shaft space.
In this rotation mechanism, just the use of bearings using mutual contact of mechanical parts, such as ceramic bearings, becomes a cause for the generation of particles. Furthermore, it is necessary to provide a large number of ceramic bearings to obtain a high air-tightness. This makes it easier for particles to accumulate in the shaft space.
Also, since the communicating pipe for discharging the particles generated from the ceramic bearings and the above magnets to the outside is formed by working the rotary shaft, the mechanical strength of the rotary shaft is reduced.
Furthermore, looking at the control of the introduction of the inert gas, since the inert gas is fed and discharged only before evacuation in the furnace, contamination of the wafer due to the generation of particles during the following processing is unavoidable.
On the other hand, in place of such bearings using ceramic or other rollers, frequent use is now being made of magnetic seal bearings using magnetic fluid (magnetohydrodynamic) since the air-tightness is high and almost no particles are generated by contact of mechanical parts.
FIG. 8A shows the overall configuration of a magnetic seal bearing unit of the related art, and FIG. 8B shows the schematic structure of the magnetic seal portion.
The magnetic seal bearing unit 100 of the related art is, as shown in FIG. 8A, provided with a seal cap 102, a magnetic seal portion 104, and a ball bearing portion 106 inside its not illustrated unit housing from the vacuum processing chamber side. A rotary shaft 108 is provided penetrating through the center. One end of the vacuum processing chamber side of the rotary shaft 108 is provided with a susceptor or a wafer holder etc., while the other end is connected to a drive.
The magnetic seal 104 comprises, as shown in FIG. 8B, a member called a pole piece 110 and a permanent magnet 112. The pole piece 110 has a plurality of ridges 110a. A plurality of spaces called magnetic seal gaps G1 to G3 are formed therebetween.
The spaces between the ridges 110a of the pole piece and the rotary shaft 108 are filled with a magnetic fluid 114 made by mixing iron oxide-based fine particles in a solvent composed of vacuum oil. The magnetic fluid 114 has a high viscosity because of its being an oil base and accumulates at the tips of the ridges 110a so as to fill the spaces between the rotating rotary shaft 108 and the ridges 110a. Also, the magnetic fluid 114 is influenced by the magnetic field resulting from the permanent magnetic 112 due to the intermixture of the magnetic material. As a result, the magnetic fluid 114 is prevented from concentrating at the lower vacuum side due to the evacuation.
FIG. 9 shows the step-shaped changes of an inner pressure of the magnetic seal gap due to the evacuation. The abscissa of FIG. 9 indicates a distance x in the axial direction of the rotary shaft, and the ordinate indicates a pressure P.
As the evacuation proceeds, air bubbles start to move in the part of the magnetic fluid closest to the vacuum chamber due to the pressure difference between the two sides and an inner pressure P1 of the magnetic seal gap G1 gradually falls. When the inner pressure P1 of the magnetic seal gap G1 becomes lower to a certain degree, air bubbles start to move in the magnetic fluid of the part second closest to the vacuum processing chamber. In the same way, air bubbles start to move in the magnetic fluid of the part third closest in a chain reaction. As a result, a step shape difference is created in the inner pressures P1 to P3 of the magnetic seal gaps G1 to G3.
Finally, as shown in FIG. 9, the inner pressure P1 of the magnetic seal gap G1 closest to the vacuum processing chamber becomes the lowest, the inner pressure P3 of the magnetic seal gap G3 closest to the drive side becomes the closest value to the air pressure Pa, and the inner pressure P2 of the magnetic seal gap G2 becomes a value between the two.
In the magnetic seal portion 104, by providing a large number of contact points with the magnetic fluid 114, the pressure difference at the two sides of the parts of the magnetic fluid 114 becomes smaller and breakage of the seal is prevented. In other words, in order not to break the seal of the parts of the magnetic fluid even when evacuating quickly from the air pressure Pa, the volumes of the seal gaps G1 to G3 and the number of the contact points are determined in advance according to the maximum exhaust capability etc. of the usable vacuum pump.
A bearing using such a magnetic seal has a ball bearing portion 106 for a mechanical support. The air-tightness is maintained at a level higher than that of the magnetic seal portion 104. Also, the particles from the ball bearing portion 106 are blocked by the magnetic seal portion 104 and not introduced into the vacuum processing chamber, therefore there is the advantage that wafers are not contaminated by the particles generated from mechanical members contacting each other.
In a bearing using a magnetic seal of the related art, however, when using dichlorosilane (SiH.sub.2 Cl.sub.2) or another gas which generates the corrosion product HCL such as in a CVD apparatus for forming a silicon nitride film, the material of the pole piece of the magnetic seal, that is, stainless steel, is corroded and sometimes ends up generating metal particles comprised of iron, chrome, nickel, etc.
Also, the vacuum oil serving as the solvent of the magnetic fluid deteriorates. When the temperature of the magnetic seal portion rises to tens of degrees to 150.degree. C. due to, for example, the heating during the processing, the vacuum oil easily becomes volatile. Therefore, the vacuum oil itself and the decomposed matter and other organic substances fly about as well. At this time, the iron oxide, manganese oxide, etc. in the magnetic fluid are fly about together with the organic matter.
The degree of contamination of a wafer by such organic matter and metals is quite low comparing with that by particles generated from mechanical bearings. Furthermore, the contamination is, in many cases, limited to the case where the reaction gas is repeatedly used.
However, due to the recent miniaturization of semiconductor devices, it has been known that even a slight amount of contaminant from the magnetic seal portion can affect the device characteristics.
For example, when forming a silicon nitride film for prevent oxidization during LOCOS formation, even a slight amount of iron contamination (1.times.10.sup.13 atoms/cm.sup.3) ruins the quality of the gate oxide film formed at the next step. As a result, the step stress time zero dielectric breakdown (TZDB) withstand voltage of a 9 nm gate oxide film sometimes declines.
Also, even slight contamination by organic matter and metals on the bear silicon region after removing the silicon nitride film leads to deterioration of the withstand voltage of the pn-junction.
As a result, it was learned that even chemical contamination of a level hardly considered a problem before could cause a drop in the yield of the devices.
On the other hand, there is an apparatus which is configured to continuously pass an inert gas to the vacuum reaction chamber (processing chamber) side of the magnetic seal. portion, however, not only is this not a fundamental solution to the wafer contamination, but also conversely spreads the contamination wider and can shorten the cycle of maintenance for cleaning and exchanging members inside the processing chamber due to the contamination spreading to the entire processing chamber.
The above disadvantages of corrosion due to a reactive gas are also seen in a wafer transfer arm operating a high vacuum. For example, Japanese Unexamined Patent Publication (Kokai) No. 9-131680 discloses the technique of protecting the mechanical portion of the wafer transfer arm used in a cluster tool etc. from the chemical atmosphere by covering the wafer transfer arm with a cover and by adjusting the pressure of the inert gas to be introduced inside the cover to, for example, higher than the surroundings to prevent entry of the reactive gas from the surroundings inside the cover.
In this publication, a magnetic seal is used as a specific example of a sealing means for increasing the air-tightness of the rotary shaft portion.
However, in the technique disclosed in this publication, the mechanism for adjusting the pressure after filling the inert gas in the cover in a wide range is complex.
Also, the higher the pressure inside the cover, the better the corrosion of the wafer transfer arm can be prevented, however, the amount of the inert gas becomes larger and the degree of vacuum of the surroundings ends up being lowered so a longer time ends up being taken for evacuation.
Furthermore, although a pressure difference is created between the inside and outside of the cover, if the set pressure inside the cover is kept down a little when it is not desired to reduce the degree of vacuum of the surroundings and lower the throughput, there is a possibility that the pressure inside the cover will become lower than the outside, for example, at the arm portion at the distal end from the inlet of the inert gas. In such a case, the magnet seal portion is exposed to the reactive gas since it is exposed to the surroundings, thus the magnetic seal portion is corroded or the vacuum oil is degraded, which results in the same disadvantage as explained above that wafer contamination spreads from the magnetic seal portion.